Fluidized bed start-up apparatus and method

ABSTRACT

An apparatus and method for supporting and starting up a fluidized bed in which a grate is disposed in a housing for receiving a single bed of particulate material, portions of which are combustible. The interior of the housing below the grate is divided into a plurality of compartments, and an air damper is disposed in each for selectively controlling the flow of air through the compartments and through the respective portions of the grate to selectively fluidize the corresponding portions of the bed of particulate material. A start-up burner is provided for igniting that portion of the combustible portion of the bed of particulate material extending above one of the compartments and additional combustible particulate material is selectively supplied to the portions of the bed.

This application is a division, of application Ser. No. 866,985, filedJan. 5, 1978, now U.S. Pat. No. 4,184,438.

BACKGROUND OF THE INVENTION

The present invention relates to fluidized beds, and more particularlyto a method and apparatus for supporting and starting a fluidized bed.

The use of fluidized beds has been recognized as an attractive means ofgenerating heat. In these arrangements, air is passed through a bed ofparticulate material, which normally consists of a mixture of inertmaterial and a fossil fuel such as coal, to fluidize the bed and topromote the combustion of the fuel. When the heat produced by thefluidized bed is utilized to convert water to steam, such as in a steamgenerator, the fluidized bed system offers an attractive combination ofhigh heat release, improved heat transfer to surfaces within the bed,and compact boiler size.

When a fluidized bed boiler is activated after a period of idle time,the bed material must be heated above the fuel ignition temperaturebefore operation can begin. However, the mass of material involved makessimultaneous start-up of the entire bed expensive and inefficient,especially with regard to relatively large beds. Although severaltechniques have been suggested for starting a fluidized bed they eachprovide less than optimum solutions. For example, since coal isdifficult to burn in a fluidized bed until the bed is raised to arelatively high temperature, the use of external burners or the like toheat the bed to the coal burning temperature often takes several hoursand is inefficient with respect to fuel use. Also, it has been suggestedto preheat the beds with the use of hot flames directed onto the topsurface of the bed. However, the high temperatures resulting from theapplication of the direct flames can injure the particles or the boilerstructure. It has also been suggested to provide physical partitionswithin the beds to divide the bed into a plurality of smaller beds, andstart up the entire bed by sequentially starting up each individual bed.However, the use of these partitions considerably adds to the cost ofthe system.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand apparatus for starting up a fluidized bed which is relativelysimple, efficient, and low in cost.

It is a further object of the present invention to provide a method andapparatus of the above type which reduces the chance of injury to thebed particles and boiler structure in addition to eliminating the costinvolved in dividing the bed into a plurality of smaller beds.

Toward the fulfillment of these and other objects the apparatus of thepresent invention comprises a housing, a grate supported in the housingand adapted to receive on its upper surface a single bed of particulatematerial at least a portion of which is combustible. Partitions areprovided for dividing the interior of the housing below the grate into aplurality of compartments. A plurality of air dampers are respectivelyassociated with the compartments for selectively controlling the flow ofair through each compartment and through the respective portions of thegrate extending over the compartments to selectively fluidize thecorresponding portion of the bed of particulate material. Burners extendin the housing for heating and igniting that portion of the combustibleportion of the bed of particulate material extending above one of thecompartments, and combustible particulate material is selectivelysupplied to the portions of the bed of particulate material.

BRIEF DESCRIPTION OF THE DRAWING

The above brief description, as well as further objects, features, andadvantages, of the present invention will be more fully appreciated byreference to the following detailed description of a presently preferredbut nonetheless illustrative embodiment in accordance with the presentinvention, when taken in connection with the accompanying drawing whichis a vertical sectional view of the apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, the reference numeral 10 refers in general toa fluidized bed boiler of the present invention consisting of a frontwall 12, a rear wall 14, and two side walls, one of which is shown bythe reference numeral 16. The upper portion of the boiler is not shownfor the convenience of presentation, it being understood that itconsists of a convection section, a roof and an outlet for allowing thecombustion gases to discharge from the boiler, in a conventional manner.

A bed of particulate material, shown in general by the reference numeral18, is disposed within the boiler 10 and rests on a perforated grate 20extending horizontally in the lower portion of the boiler. The bed 18can consist of a mixture of discrete particles of inert material andfuel material such as bituminous coal.

An air plenum chamber 22 is provided immediately below the grate 20 anda pair of partitions 24 and 26 divide the upper portion of the chamber22 into three compartments 28, 30, and 32. An air inlet 34 is providedthrough the rear wall 14 in communication with the chamber 22 fordistributing air from an external source (not shown) to the chamber.

A pair of air dampers 36 are provided in the inlet 34, and in each ofthe compartments 28, 30, and 32 for controlling the flow of air into thechamber and through the compartments. The dampers 36 are suitablymounted in the inlet 34 and in the compartments 28, 30, and 32 forpivotal movement about their centers in response to actuation ofexternal controls (not shown) to vary the effective openings in theinlet and the compartments and thus control the flow of air through theinlet and the compartments. Since these dampers are of a conventionaldesign they will not be described in any further detail.

Three air preheat burners 40, 42, and 44 are mounted through the frontwall 12, the side wall 16 and the rear wall 14, respectively, andcommunicate with the chambers 28, 30, and 32, respectively, forpreheating the air flowing through the compartments. A bed light-offburner 46 is mounted through the front wall 12 immediately above thegrate 20 for initially lighting off a portion of the bed 18 duringstart-up. The details of the operations of the burners 40, 42, 44, and46 will be described later.

A distributor, shown in general by the reference numeral 50, is mountedrelative to the upper portion of the front wall 12 and operates todistribute particulate fuel material, such as coal, into selectedportions of the bed 18 during start-up. A pneumatic spreader is used forexample, however, the distributor could also be mechanical. Thedistributor 50 includes a inlet pipe 52 for receiving the coal andfeeding same, by gravity onto a distributor tray 54 which extends intothe interior of the boiler 10. The tray 54 is pivotally mounted relativeto a actuating lever 56 for controlling the movement of the tray betweenthe positions shown by the solid lines and the two positions shown bythe dashed lines. A control for the pivotal movement of the tray 54 isshown in general by the reference numeral 58 and can be of anyconventional type. The distributor 50 also includes an air distributorunit, shown in general by the reference numeral 60 for distributing airat a selected rate through a plurality of vanes, one of which is shownby the reference numeral 62, to inject the air across the coal on thetray 54. As a result the coal is distributed into portions of theinterior of the boiler 10 that are determined by the position of thetray 54.

A plurality of feeders 64, 66, and 68 are provided through the side wall16 immediately above the bed 18 at spaced intervals which correspond tothe spacing between the compartments 28, 30, and 32 in the air chamber22. The feeders 64, 66, and 68 also are adapted to introduce and feedparticulate coal to the bed 18.

As a result of the location of the compartments 28, 30, and 32 and thefeeders 64, 66, and 68, the bed 18 is effectively separated into threeportions shown in general by the reference numerals 70, 72, and 74,respectively for reasons that will be set forth in detail later.

To start-up the bed 18, the dampers 36 associated with the air inlet 34and the dampers 36 associated with the compartment 28 are opened, whilethe dampers 36 associated with compartments 30 and 32 are closed. Air isthus distributed upwardly through the compartment 28 and through theperforations in the grate 20 immediately above the compartment 28 andinto the bed portion 70. This loosens the particulate material in thebed portion 70 and reduces material packing and bridging.

The dampers 36 associated with the compartment 28 are then closed andthe dampers 36 associated with the compartment 30 are opened tointroduce air into the bed portion 72 in a manner similar to thatdiscussed above. The dampers 36 associated with the compartment 30 arethen closed and the dampers 36 associated with the compartment 32 areopened to repeat the process with respect to the bed portion 74.

The dampers 36 associated with the compartment 32 are then closed andthe dampers 36 associated with the compartment 28 are opened and the airpreheat burner 40 is fired. The air thus passes through the compartment28, and is thereby preheated before it passes upwardly through the bedportion 70, with the flow rate of the preheated air being controlled topermit fluidization of the bed portion without substantial materialelutriation.

When the temperature of the bed portion 70 reaches a predeterminedvalue, such as 250°-300° F., the light-off burner 46 is fired to furtherheat the material in the bed portion 70. When the temperature of the bedportion 70 reaches a predetermined higher lever, such as 800°-950° F.,the distributor 50 is activated in a manner to move the tray 54 to itslowermost position as shown by the dashed lines, and to turn on the airdistributor 60 to distribute the particulate fuel from the inlet pipe 52into the upper portion of the bed portion 70.

When the fluidized bed portion 70 reaches a further elevatedpredetermined temperature, such as 1200° F., the dampers 36 associatedwith the compartment 30 are opened and the air preheat burner 42 isfired so that the bed portion 72 is fluidized with preheated air. Due topresence of the preheated air, and the lateral mixing of the bed portion72 with the bed portion 70, the temperature of the bed portion 72 willrise rapidly and, when it reaches approximately 800°-900° F., theposition of the tray 54 of the distributor 50 will be adjusted toinclude distribution of the particulate coal to the upper portion of thebed portion 72 in addition to the bed portion 70, as described above.When the fluidized bed portion 72 reaches the predetermined elevatedtemperature, which in the above example is 1200° F., the bed portion 74is fluidized by opening the dampers 36 associated with the compartment32 and firing the air preheat burner 44. The position of the tray 54 isthen adjusted so that the particulate coal is also distributed to thebed portion 74.

After all three of the beds have been fluidized and have reached thepredetermined elevated temperature of 1200° F. in the foregoing manner,the air preheaters 40, 42, and 44, as well as the light-off burner 46are turned off and the individual feeders 64, 66, and 68 extendingimmediately above the bed portions 70, 72, and 74, respectively, can beactivated to distribute particulate fuel directly to the upper portionof these beds. When the these fluidized bed portions reach a finaldesired temperature, such as 1550° F., the relative precise control ofthe feed rate and accuracy of the distribution of the fuel materialafforded by the distributor 50 is no longer needed, and the latter canbe shut down and the final bed temperature can be controlled bycontrolling the rate of feed of the feeders 64, 66, and 68.

It is thus seen that the present invention provides an effective yetsimple method of starting up a fluidized bed with a minimum of damage tothe particulate material, while avoiding the costs associated with apartitioned type boiler.

It is understood that if the boiler is used for the purpose of steamgeneration, a plurality of heat exchange tubes carrying the fluid to beheated, such as water, will be routed through the interior of the boilerin a conventional manner with these tubes being omitted in the drawingfor the convenience of presentation.

A latitude of modification, change and substitution is intended in theforegoing disclosure and in some instances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that the appended claims beconstrued broadly and in a manner consistent with the spirit and scopeof the invention herein.

What is claimed is:
 1. A method for starting a fluidized bed, comprisingthe steps of supporting a bed of particulate material at least a portionof which is combustible, dividing the area below said bed into aplurality of zones, selectively passing air through each zone andthrough the corresponding portions of said bed extending over said zonesto selectively fluidize said bed portions, individually controlling theflow of air through each of said zones and therefore through said bedportions, preheating the air in each zone as it flows through saidzones, igniting the combustible portion of the particulate materialextending above one of said zones, and supplying additional combustibleparticulate material to said bed portions.
 2. The method of claim 1wherein said steps of supplying additional combustible particulatematerial to said bed portions comprises the steps of supplyingcombustible particulate material from a single source to said bedportions during start-up of said bed, and supplying combustibleparticulate material from individual sources extending over said zonesto said portions of said bed after start-up of said bed.
 3. The methodof claim 1 further comprising the step of repeating said air passingstep, the first air passing step being for the purpose of reducingmaterial packing and material bridging and the second air passing stepbeing done in conjunction with said step of preheating.
 4. A method forstarting a fluidized bed, comprising the steps of supporting a bed ofparticulate material at least a portion of which is combustible,selectively introducing the flow of air to successive portions of saidbed of particulate material to selectively fluidize said portions ofsaid bed of particulate material, igniting the combustible portion ofone of said bed portions to start-up said bed, supplying additionalcombustible particulate material to said bed portions from a singlesource during start-up of said bed, and then supplying additionalcombustible particulate to said bed portions from a plurality ofindividual sources respectively associated with said bed portions afterstart-up of said bed.
 5. The method of claim 4 further comprising thesteps of individually controlling the flow of air through each of saidzones and therefore through said bed portions.
 6. The method of claim 4further comprising the step of repeating said air passing step, thefirst air passing step being for the purpose of reducing materialpacking and material bridging and the second air passing step being donein conjunction with said step of preheating.